Digital cameras and other digital imaging devices generate machine-readable image data (sometimes referred to simply as “image data”) representative of images of objects. The process of generating image data representative of an image of an object is sometimes referred to as imaging or capturing an image of the object. During the imaging process, a digital camera forms an image of the object onto a two-dimensional photosensor array. The photosensor array has a plurality of discrete photodetector elements that are sometimes referred to as photodetectors. Each of the photodetectors generates an electrical signal having values proportional to the intensity of light incident on the photodetectors. The output of the photosensor array is connected to an analog-to-digital converter that is used to convert the electrical signals generated by the photodetectors into digital numbers. The digital numbers output from the analog-to-digital converter are proportional to the intensity of the light incident on the photodetectors. These digital numbers are sometimes referred to as counts or raw data. The raw data consists of numbers wherein a high number is typically representative of a photodetector that received bright light and a low number is typically representative of a photodetector that received dim light.
In color digital photography, color is typically generated using filters in a prescribed color-filter array pattern. A filter placed over the top of each photodetector limits the response of the photodetector so that the raw data produced is limited to a preselected wavelength of light. These preselected wavelengths of light typically correspond to the three additive primary wavelengths or colors of red, green, and blue. The raw data representative of three colors is processed to generate one pixel in the final image. One common type of color filter uses a Bayer pattern. The Bayer pattern is a four-pixel cluster that consists of a pixel that responds to red light, two pixels that respond to green light, and a pixel that responds to blue light.
Digital images are generated by sampling a continuous scene or object. The sampling process consists of mapping the scene onto to the two-dimensional grid of photodetectors that forms the photosensor array. Due to the discrete nature of the digital imaging process the image generated by a digital camera is subject to certain image anomalies resulting from the sampling process. One anomaly is aliasing, which is the generation of false frequencies when an image is undersampled. Aliasing becomes more apparent when an image of an object having high spatial frequency content is imaged.
The highest spatial frequency that may be replicated is one half the sampling frequency, which is referred to as the Nyquist frequency. Frequencies higher than the Nyquist frequency are aliased down to lower frequencies. The lower frequency features introduce artifacts into the image, which can create false images and form moiré patterns in periodic scenes. The anomalies are even greater if the digital camera is used to generate a video or movie images because the camera typically does not use all the photodetectors in the array. Thus, the sampling rate decreases and the effects due to aliasing increase.